Gallium nitride was first grown by H. P. Maruska and J. J. Tietjen, Appl. Phys. Lett., 15, 327 (1969) using hydride vapor phase epitaxy (HVPE). It employs gaseous hydrogen chloride to flow over and pick up metallic gallium and produce gallium chloride. The gallium chloride is then reacted with ammonia to form single crystal gallium nitride on a substrate. Today, this is the preferred method for growing single crystal gallium nitride, the native substrate for the growth of the metal nitrides.
There has been a concerted effort by several groups to produce single crystal GaN by many methods including the aforementioned HVPE method, ammonothermal growth, U.S. Pat. Nos. 6,656,615, 6,398,867, and 6,177,057, and flux growth, U.S. Pat. No. 6,592,663, and single crystal AlN has been produced by sublimation growth. There have also been several methods to produce GaN and AlN powder such as E. Ejder, et al., J. of Cryst. Growth, 22, 44 (1974).
Despite the large number of processes developed for gallium nitride and aluminum nitride single crystals and powders, there have been relatively few efforts in producing dense polycrystalline GaN. There has been some concentrated effort on producing polycrystalline AlN for heat sinks because of the high thermal conductivity of AlN. However, state of the art AlN heat sinks have high levels of impurities, especially oxygen, due to the difficulty in sintering AlN into high-density forms from high-purity AlN powder because of low self-diffusion rates in strongly covalently-bonded nitrides such as AlN and GaN. Therefore, oxygen-containing or other impurity-containing compounds are added to aid in sintering, and temperatures greater than 1800° C. are employed to achieve bodies of nearly full density.
It is known in the art that oxygen reduces thermal conductivity in the nitrides that are desirable for heat sinks. Furthermore, InN and GaN start decomposing at high temperatures, which prevents them from being sintered without the addition of substantial nitrogen overpressures. The processes described above to produce single crystal nitride substrates (excluding HVPE, which uses Ga metal) would also benefit from an inexpensive source of high-purity, low-cost, high-density polycrystalline GaN. Furthermore, by doping polycrystalline group III nitrides such as InN, GaN, AlN and the corresponding alloys with rare earth metals and transition metals such as chromium and titanium, ceramic solid state laser hosts can be constructed that would have improved performance over current ceramic laser hosts. Finally, thin deposition techniques such as laser ablation and sputtering would benefit high-purity, dense, commercial nitride sputtering targets which are not currently available.
D'Evelyn et al. in U.S. Pat. No. 6,861,130 employed hot pressing of GaN powder to obtain polycrystalline GaN with commercial GaN powder. It is now known in the art that the majority of commercial powder has a high level of oxygen impurities, which most likely aid in the densifying of the polycrystalline GaN. Use of this process with high purity GaN powder would produce lower density polycrystalline GaN and the high pressures used limits the practicability of this method for commercial uses.
U.S. Pat. Nos. 6,406,540 and 6,113,985 describe a method using ammonium halides to produce several forms of GaN, including polycrystalline, but the polycrystalline material was in the form of a thin crust that formed on the gallium metal or on a substrate that appeared to be much less than 1.0 mm in thickness (see FIG. 6). Argoita, et al., Appl. Phys. Lett., 70, 179 (1997) also formed a thin layer of polycrystalline GaN with a nitrogen plasma. The difficulty of using the above techniques to produce a polycrystalline slab of GaN in which a nitrogen-containing gas is flowed over molten Ga is that a nearly 100% dense GaN crust forms on the surface of the molten Ga, which then blocks further contact of the gas with the Ga metal and ends the process with a relatively low yield.
Several groups have used the sublimation process to grow high purity polycrystalline AlN, but sublimation is conducted at very high temperatures and extremely expensive crucibles are required, which have a shelf life of only several hundred hours. Therefore, no inexpensive, reliable method for producing high-purity polycrystalline III-nitride, including GaN, in bulk or slab form has been reported. There is accordingly a need and market for a new method that avoids these shortcomings to produce high-density, high-purity, large-area, bulk polycrystalline III-nitrides.